1. Field of the Invention
The present invention relates to counterbalanced systems capable of measuring rotation rates about multiple inertial axes. More particularly, this invention pertains to a system of the above type wherein counteroscillating rotors are electromagnetically driven.
2. Description of the Prior Art
Systems employing paired counteroscillating rotors have been utilized as relatively low-cost approaches to the measurement of rotation rates, and, in some cases, linear accelerations with respect to multiple inertial axes. For example, a system for measuring rotation rates (and accelerations) with respect to a set of three orthogonal axes utilizing a single drive mechanism is taught in a series of U.S. patents of Stewart and Fersht (U.S. Pat. No. 4,996,887 entitled xe2x80x9cThree Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillatorxe2x80x9d; U.S. Pat. No. 5,007,289 entitled xe2x80x9cThree Axis Inertial Measurement Unit With Counterbalanced, Low Inertial Mechanical Oscillatorxe2x80x9d; and U.S. Pat. No. 5,065,627 entitled xe2x80x9cThree Axis Inertial Measurement Unit With Counterbalanced, Low Inertial Mechanical Oscillator.xe2x80x9d) The system disclosed in each of those patents employs a drive mechanism comprising of plurality of vanes having piezoelectric strips mounted thereon for affecting deflections that cause a pair of counterbalanced platforms to oscillate out-of-phase about a common axis within a housing or case. Closed-loop accelerometers are mounted about the peripheries of the platforms and oriented at predetermined angles with respect to the axis of oscillation that permit determination of angular rates, through measurement of the Coriolis accelerations, about three orthogonal axes as well as linear accelerations with respect to those axes.
Other devices are disclosed in U.S. Pat. No. 5,932,803 of Wyse entitled xe2x80x9cCounterbalanced Triaxial Multisensor With Resonant Accelerometersxe2x80x9d, U.S. Pat. No. 5,932,805 of Stewart entitled xe2x80x9cMultisensor With Directly Coupled Rotorsxe2x80x9d and U.S. Pat. No. 6,041,652 of Stewart entitled xe2x80x9cMultisensor Rotor Flexure Mounting.xe2x80x9d Each of the above devices employs an arrangement of radially-directed drive beams for supporting a coaxial pair of rotors. In each case, the mechanism for driving the counteroscillating rotors or platforms relies upon inducing bending in the vanes by application of voltages to strips of piezoelectric material fixed to their opposed surfaces. The devices disclosed in those applications employ hinged accelerometers that are positioned at the ends of the rotor arms and oriented at angles to the planes of dither of the paired rotors to enable the devices to measure Coriolis and linear accelerations with respect to the three orthogonal inertial axes.
U.S. Pat. No. 5,428,995 of Fersht, Wyse, Stewart and Miller entitled xe2x80x9cCounterbalanced Vibratory Triaxial Angular Rate Sensor With Open Loop Outputxe2x80x9d teaches a device for open loop measurement of rotation rates with respect to three orthogonal axes in which the arms of counteroscillating rotors are configured as open loop accelerometers. In each case, a pendulous mass is held at the end of a rotor arm formed of parallel surfaces, each of which comprises a composite of metallization and piezoelectric material layers in which the metallizations act as electrodes. As in the case of the devices described in the preceding paragraph, radial vanes, which support the rotors within surrounding frames, include strips of piezoelectric material at their opposed surfaces whereby the vanes may be induced to deflect in such a way as to cause desired oscillatory movements of the rotors.
Certain of the features common to the devices of the above-described type that employ counteroscillating rotor pairs introduce problems in terms of both performance and manufacture. By employing strips of piezoelectric material at opposed sides of driving vanes, one must attain great precision to assure that the overall device remains highly symmetric since piezoelectric material expands or contracts in all directions when a voltage is applied. Therefore, generated forces act in all directions, making it absolutely critical that a high degree of symmetry be obtained when mounting the piezoelectric elements to avoid the introduction of motion along the direction of the dither axis. Such motion will be identical to those motions caused by Coriolis accelerations under measurement. Gyro bias is highly sensitive to the manner in which the piezoelectric material is affixed to the multiple radial vanes that drive the rotors as out-of-plane forces are a major contributor to bias in a piezoelectric dither mechanization. Such out-of-plane forces may also be induced by twisting of the drive vanes which must therefore be relatively stiff, introducing stresses from radial stressing.
In addition, the forces required to maintain a given dither amplitude are proportional to the Q of the dither mode of vibration. The mounting of piezoelectric elements to the radial vanes decreases the Q of the resonant mode. As a result, higher driving forces are required, which, in turn, results in higher bias. Further, due to the complex and labor-intensive processes required for wiring multiple piezoelectric elements, such devices may be rendered uneconomical.
The mounting of accelerometers at an angle in the multisensor devices described above significantly complicates their manufacture and operation. Misalignment errors are critical in such devices as this may lead to the pickup of dither motion. In addition, angular mounting of accelerometers requires the use of complex geometrical transformation for evaluation of sensor output.
The present invention addresses the preceding and other shortcomings of the prior art by providing, in a first aspect, a sensor that includes a first rotor and a second rotor, each including a plurality of radially-directed rotor arms that extend from a central hub. Each of the rotors is received within an associated frame. A plurality of vanes connects each of the hubs to its associated frame. A torsion spring is fixed to the hub.
Each of the rotor arms comprises parallel upper and lower radially-directed planar surfaces and an orthogonal end surface. At least one accelerometer is fixed to at least one of the upper, lower and end surfaces.
In a second aspect, the invention provides a sensor that includes a first rotor and a second rotor, each including a plurality of radially-directed rotor arms that extend from a central hub. Each of the rotors is received within an associated frame. A plurality of vanes connects each of the hubs to its associated frame. A torsion spring is fixed to the hub.
An electromagnet is fixed to the interior of each of the frames. One of the rotor arms of each rotor includes a region of magnetic composition. Each rotor and associated frame is arranged so that the region of magnetic composition of a rotor arm is substantially adjacent the electromagnet. At least one accelerometer is fixed to at least one of the rotor arms.
In a third aspect, the invention provides a sensor having a first rotor and a second rotor, each being substantially planar and including a plurality of radially-directed rotor arms that extend from a central hub. The rotors are received within a frame. A plurality of vanes connects each of the hubs to the frame so that the rotors are substantially parallel to one another. A torsion spring is fixed to the hubs.
Each of the rotor arms comprises parallel, radially-directed planar upper and lower surfaces and an orthogonal end surface. At least one accelerometer is fixed to at least one of the upper, lower and end surfaces. The rotors are driven to counteroscillate about the torsion spring.
In a fourth aspect, the invention provides a sensor having a first rotor and a second rotor, each being substantially planar and including a plurality of radially-directed rotor arms that extend from a central hub. The rotors are received within a frame. A plurality of vanes connects each of the hubs to the frame so that the rotors are substantially parallel to one another. A torsion spring is fixed to the hubs.
A pair of electromagnets is fixed to the interior of the frame. One of the rotor arms of each rotor includes a region of magnetic composition. Each rotor is arranged so that a region of magnetic composition of a rotor arm is adjacent the electromagnet. At least one accelerometer is fixed to at least one of the rotor arms.
The various features and advantages of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the various features of the invention. Like numerals refer to like features throughout both the drawing figures and the written description.